(a) Technical Field
The present invention relates to a transmission oil composition for use in a push belt continuously variable transmission (CVT). More particularly, the present invention relates to a push belt CVT oil composition including a hydrogenated base oil or synthetic base oil as well as a viscosity control agent, a dispersing agent, a friction control agent and other additives, which provides improved friction characteristics, durability, thermal stability, oxidation stability and transmission performance, can be used in various temperature ranges, and can improve transmission efficiency (fuel efficiency).
(b) Background Art
In general, a transmission is provided between a clutch and a propeller shaft or between a clutch and final reduction gears to provide speed and torque conversions of the power from an engine suitable for the driving state of an automobile to the drive wheels. It is often classified into a manual transmission and an automatic transmission. The automatic transmission refers to a transmission whereby the speed and torque of the engine are converted automatically without the need of the driver's manual clutch operation and gear shifting according to the driving condition as in the manual transmission.
In order to improve transmission efficiency, multi-step automatic transmissions have been developed such as 6-speed, 7-speed, 8-speed, etc. Recently, the continuously variable transmission (CVT) is adopted increasingly.
The push belt CVT was developed by Dr. Van Doore and is adopted in many cars. The CVT allows stepless transmission through belt-pulley variation. The variable pulley presses the belt with an adequate pressure, and the gear ratio is determined by the ratio of the pressure applied to the first pulley and the second pulley. An adequate friction needs to be maintained between the pulley and the belt to avoid slippage.
As such, since an adequate friction needs to be maintained between the pulley and the belt, it is necessary to reduce wearing by friction using gears and bearings. And, when the torque converter is combined with a planetary gear system, the friction material used for the clutch should have superior friction characteristics and there should be no shifting shock during transmission.
Accordingly, focuses are given to the improvement of friction characteristics and other physical properties for transmission oil used in the CVT.
As previously known transmission oils, Japanese Patent Application Publication No. 2009-0286831 discloses a lubricating oil composition for a metal belt-type CVT comprising a base oil as well as 0.005-0.1 wt % of a phosphorus compound and 0.01-20 wt % of an ashless dispersant, an antioxidant, an extreme pressure agent, a friction control agent, a viscosity index improver, etc. as additives. Japanese Patent Application Publication No. 2001-0323292 discloses a lubricating oil composition for a push belt-type automatic transmission comprising a lubricating base oil as well as (A) an organic acid metal salt (100-1000 ppm based on the metal content), (B) an anti-wear agent and (C) a boron-containing succinimide, wherein the metal salt of the organic acid having a long-chain hydrocarbon group is selected from salicylate, carboxylate, sulfonate, phosphonate, etc. of Ca or Mg, and further comprising 0.01-5 wt % of a friction control agent, 0.1-10 wt % of an imide-based ashless dispersant, 0.05-5 wt % of an antioxidant and 1-7 wt % of a polymethacrylate viscosity index improver as additives.
However, these transmission oil compositions have physical properties, especially friction characteristics, improper to be used as the push belt CVT oil.
In order to solve this problem, Korean Patent Application Publication No. 2005-0028808 presents a power transmitting fluid comprising a base oil as well as an additive composition comprising 0.1-10 wt % of a dispersing agent such as succinimide, 0.1-3.0 wt % of an antioxidant and 0.01-1.0 wt % of an antifoaming agent as additives and further comprising a sulfur-based extreme pressure agent, a friction control agent, a viscosity index improver, etc. The additive composition acts as a phosphorous source for providing improved wear resistance and as a friction controller for providing improved anti-vibration durability. It is described that the power transmitting fluid comprising the additive composition provides improved wear resistance and improved anti-vibration durability. Although use of the power transmitting fluid for the push belt-type transmission is presented, there remain a lot to be improved in terms of friction characteristics, thermal stability, durability, or the like.